The square within a square has a different spectrum of
meaning than . It basically means keep, retain, keep inside, or close in. When used as a British boy-scout sign
it means wait here.
In some cartographical systems it stands for fortress (see the Word Index for
It can also represent a manhole, as it does in electrical engineering. A manhole is an opening to a lower, closed space or room where lines can be checked and repaired.
On electrical household appliances means that they are better isolated than usual. The sign indicates appliance that can use both grounded and non-grounded sockets or outlets.
The similar is a sign for pushbutton for door opening in some modern contexts.